1. Field of the Invention
The invention relates generally to vacuum formed and pressure formed products and processes, particularly hard sided luggage.
2. Background Art
Hard sided luggage usually consists of two shells, commonly called a lid shell and a base shell. Each shell typically is made from a sheet of thermoplastic material that has been molded in the shape of a container. The shell frequently is shaped as a rectangular box with rounded corners whose open side is defined by a peripheral edge. The peripheral edges of the two shells substantially correspond to one another, so that the lid shell may be placed, concave side down, upon the base shell with its concave side up, so that the respective edges are aligned and/or in contact. So arranged, the shells may then be connected by hinges and releasable latches, as known in the art, to define a container with an accessible interior space.
A principal objective in hard sided luggage making is to provide a case that is at once both lightweight and strong. Besides being puncture-proof and unbreakable, a hard sided case of ideal strength also resists gross deformation of its overall shape due to external torsional or flexural forces. While some minor flexibility is desirable in a hard sided case, an advantage of hard sided luggage should be its rigidity--the ability to withstand forces without undue amounts of twisting or bending. Twisting of any type of luggage can jeopardize the luggage contents, and may also damage hinges and promote latch failure.
Shells incorporated in hard sided luggage are normally molded using one of two manufacturing processes. Injection molding involves the injection, under pressure, of molten thermoplastic resin, typically polypropylene, into massive steel molds to form substantially complete shells including integral frame and attachment points for wheels, handles, etc. The nature of injection molding processes and machinery limit somewhat the ultimate configuration of the molded item. Injection molding also requires the use of very high pressure systems, which can be expensive to acquire and maintain, and which may limit product variety and rapid product modification.
The other main process, commonly called "vacuum forming," involves forcing a heated sheet of thermoplastic against a male or female mold. The driving force is provided by a pressure differential, so that a difference in air pressure on opposite sides of the sheet causes the sheet to move against the mold. Strictly speaking, "vacuum forming" refers to the creation of a "negative," or reduced pressure in the volume between the sheet and the mold, thereby pulling or "sucking" the sheet up to the mold. Alternatively, "pressure molding" involves the creation of a volume of "positive," or elevated pressure on the side of the sheet opposite from the mold, thereby blowing or pushing the sheet to the mold. Moreover, pressure molding and vacuum molding can and frequently are simultaneously performed within a single apparatus, and such combined processes sometimes are generically referred to as "vacuum molding." Unless the specific terms "vacuum forming" or "pressure forming" are used, this disclosure shall use the generic term "pressure differential forming," meaning vacuum forming alone, pressure forming alone, or a combination of the two processes.
In differential pressure forming, after the pliable heated sheet has conformed to the shape of the mold surface, it is removed from the mold. The three dimensional shaped shells are then trimmed to proper size and to eliminate edge sections (sometimes called offal or selvage portions) needed for the process but not forming a part of the final product. Conventionally, in order for the resulting formed case to have adequate rigidity, the formed shell is riveted or stapled to a separate frame component--commonly a metal frame extruded from aluminum or magnesium alloys. Hinges are attached to the frames, and other hardware and lining elements are then attached to form the completed case.
A disadvantage of the standard differential pressure forming processes for making luggage is the need for the separate frame component to be attached to the shell. A typical differential pressure formed shell, alone, is relatively crush proof and puncture proof, has flexible strength yielding to stress, and is subject to both plastic and elastic deformation when subjected to any considerable loading. The frame, on the other hand, is light and strong, but comparatively inflexible. The stiffness of the frame and the flexibility of the shell do not compliment each other, especially at the local stress points where the shell and the frame are riveted or stapled together. At these points of elevated stress, rivets or staples are prone to pull through or tear the thermoplastic shell. Also, the magnesium and aluminum frames add expense to the manufacture of the case; the frame itself usually requires a number of finishing steps because it is a principal aesthetic feature of the completed article of luggage. Also, the frame is typically used to hide the raw edge of the trimmed formed shell.
Another problem frequently encountered in conventional vacuum-forming container manufacturing processes is undesirable thinning in the walls of the finished product, particularly near corners and edges. This poses serious disadvantages to container strength, since corners and edges tend to be the portions of the container subject to elevated impact and bending stresses.
Still another problem encountered in the present art is the limitations that are imposed upon the shape of the final product by the need to remove the finished product from the mold. The finished product is simply pulled straightaway from the mold once the thermoplastic has cooled. In order for this separation to be accomplished without cutting the molded product, the mold must be shaped to provide that no part of the molded product interlocks with the mold itself; if a standard vacuum mold surface has substantial projections or depressions into which product material is forced, a the product will interlock with the mold to prevent the product from being pulled away from the mold in the direction of attempted separation. Unfortunately in the present art, this limitation on the shape of the mold has hindered container design, including the design of integrally framed luggage shells.
Luggage shells have also been manufactured using blow molding and rotational molding processes. Blow molding and rotational molding have a variety of limitations in luggage applications. For example, blow molding and rotational molding processes have poor thickness control, resulting in thin spots in walls. These prior art processes are also limited as to the shapes and compositions of the products produced, and do not permit ready lining or finishing of interior surfaces.
Thus a need remains for a luggage product manufactured using the comparatively simple and inexpensive pressure differential molding process, but which overcomes the disadvantages of present products and improves upon the manufacturing process. Against this background, the present invention was developed.